Electrodes for electrolysis using valve metals such as Ti, etc., as a substrate are used as excellent insoluble metal electrodes in a variety of electrochemical fields. In particular, they have been widely put to practical use as chlorine-generating anodes in electrolysis of sodium chloride. Such metals includes Ti as well as Ta, Nb, Zr, Hf, V, Mo, W, etc.
These metal electrodes generally comprise metallic titanium coated with various electrochemically active substances such as platinum group metals or oxides thereof, as typically disclosed, e.g., in U.S. Pat. Nos. 3,632,498 and 3,711,385. They are designed to retain a relatively low chlorine overpotential, for particular use as electrodes for generation of chlorine.
However, when these metal electrodes are used for oxygen generation or as an anode in electrolysis accompanied by oxygen generation, the overpotential at the anode gradually increases. In extreme cases, passivation of the anode occurs, ultimately resulting in failure of continuation of the electrolysis. Such passivation of the anode appears to arise mainly from reaction of the Ti substrate with oxygen from the oxide coating of the electrode itself, or with oxygen from the electrolytic solution diffused and permeated through the electrode coating, to thereby form titanium oxide, that is a poor conductor. Further, since the poor conductor oxide is formed at the interface between the substrate and the electrode coating, it causes the coating to peel off, ultimately destroying the electrode.
Electrolytic processes wherein the anode product is oxygen, or evolution of oxygen occurs as a side reaction, are involved in many industrially important fields, and include electrolysis using a sulfuric acid bath, nitric acid bath, an alkaline bath, etc.; electrolytic winning of Cr, Cu, Zn, etc.; various electroplating processes; electrolysis of a diluted saline solution, sea water, hydrochloric acid, etc.; organic electrolysis; electrolytic production of chlorates; and the like. However, the above-described problems have created problems in the application of the conventional metal electrodes to these fields.
In order to solve such problems, it has been proposed to provide a barrier composed of a Pt-Ir alloy or an oxide of Co, Mn, Pd, Pb, or Pt between the conductive substrate and the electrode coating, in order to prevent passivation of the electrode due to oxygen permeation, as disclosed in Japanese Patent Publication No. 19429/76.
Although such an intermediate barrier is somewhat effective to prevent diffusion and permeation of oxygen during electrolysis, the material composing the barrier per se possesses a considerable electrochemical activity so that it reacts with an electrolyte permeated through the electrode coating to form electrolytic products such as gases on the surface of the barrier. Such electrolytic products physically and chemically impair adhesion of the electrode coating, creating a potential problem that the electrode coating will fall off before the expiration of the life of the electrode coating. In addition, the barrier has a problem of corrosion. Therefore, this proposal is still unsatisfactory for attaining sufficient durability of electrodes.
Another approach is an electrode having a laminated coating comprising a layer of an oxide of Ti, etc., and a layer of a platinum group metal or its oxide, as taught in Japanese Patent Publication No. 48072/74. However, when such an electrode is used for electrolysis involving oxygen evolution, passivation similarly takes place.
In an attempt to overcome these disadvantages, an electrode having an intermediate layer comprising an oxide of Ti or Sn and an oxide of Ta or Nb in which Pt may be dispersed, as disclosed in Japanese Patent Publication Nos. 22074/85 and 22075/85, was previously developed by one of the present inventors along with others. These electrodes exhibit excellent conductivity and durability sufficient for practical application. Nevertheless, since the intermediate layer is formed by thermal decomposition, there remains room for further improvement with respect to denseness of the intermediate layer in order to enhance durability of the electrode.